Recently, as smart phones and tablet PCs are spread out and high-capacity multimedia communication is activated, mobile traffic is suddenly increased. It is expected that in the future, mobile traffic will be increased about two times every year. Most of such mobile traffic is transmitted through an eNB, and thus communication service providers are facing with a severe network load problem. In order to handle the increasing traffic, communication service providers are increasing network equipment and have commercialized next-generation mobile communication standards capable of efficiently processing a large amount of traffic, such as mobile WiMAX and Long Term Evolution (LTE). However, another solution is necessary to handle the amount of traffic that will be further suddenly increased in the future.
Device-To-Device (D2D) communication is a distributive communication technology in which traffic is directly transferred between adjacent nodes without using infrastructure, such as an eNB. In a D2D communication environment, each node, such as a portable terminal or UE, autonomously discovers another piece of UE that is physically adjacent to the node, sets up a communication session, and then transmits traffic. D2D communication has been in the spotlight as an element technology of a next-generation mobile communication technology subsequent to 4G because a traffic overload problem can be solved by distributing traffic concentrated on an eNB as described above. For such a reason, a standard organization, such as 3GPP or IEEE, is pushing ahead with the enactment of D2D communication standards based on LTE-A or Wi-Fi. Qualcomm, etc. is developing an independent D2D communication technology.
It is expected that D2D communication may contribute to an increase in the performance of a mobile communication system and also create new communication services. Furthermore, D2D communication can support services, such as social network services or network game based on adjacency. As described above, it is expected that the D2D technology may provide new services in various fields.
The D2D communication technology that has already been widely used includes infrared communication, ZigBee, radio frequency identification (RFID), and Near Field Communication (NFC) based on RFID. In a strict sense, however, such technologies are difficult to be classified as D2D communication technologies in which traffic of an eNB is distributed because the technologies support only communication for special purposes within a very limited distance (about 1 m).